In handling analog signals, duplexing methods and apparatus have been used rather extensively to minimize the number of transmission lines and increase transmission capacity. Thus, for example, in telephone art, methods entailing use of bridge circuits operated by voice signals have been in use extensively to provide full duplex operation. Also, modulation approaches of various nature have been utilized in radio or high frequency transmission art to provide a full duplex operation. It has been found that these approaches are slow and rather complex. For example, the modulation approach tends to be speed limited because the carrier has to be faster than the signal and requires rather complex modulation, demodulation and detection circuitry.
Serious attempts have been made to utilize the aforementioned duplexing methods or apparatus or develop new methods or apparatus in handling digital signals. However, to date, as far as the present inventor is aware, such attempts have been successful only to the extent of providing full duplex operation using frequency shift modulation or similar techniques that require sending carrier signals at higher frequencies than the data rate. This requires complicated encoding and decoding circuitry and restricts the data transmission rates to well below the speeds of modern digital circuits. Present techniques for sending high speed digital data in a signal line are restricted to half duplex operation.